Process for the preparation of paclitaxel

ABSTRACT

A process for the preparation of paclitaxel starting from 10-deacetylbaccatine III.

The present invention relates to a process for the preparation ofPaclitaxel.

Paclitaxel is a molecule of natural origin having wide spectrumantitumor activity, with the following structural formula:

The compound, first recovered from Taxus brevifolia bark and from othernatural sources, can be prepared semi-synthetically according to anumber of procedures described in both scientific and patent literature.

U.S. Pat. No. 4,924,011 discloses the semi-synthesis of paclitaxel using10-deacetylbaccatine III protected at the C-7 hydroxyl with atrialkylsilyl group and subsequently acetylated at C-10. The resultingintermediate is reacted with(2R,3S)-N-benzoyl-2-O-(1-ethoxyethyl)-3-phenyl-isoserine and theresulting product is deprotected to give paclitaxel.

WO-93/06094 discloses the preparation of paclitaxel by reacting aβ-lactam precursor with 7-O-triethylsilyl-baccatine III, followed bymild acid hydrolysis.

According to U.S. Pat No. 5,476,954, paclitaxel is prepared startingfrom 10-deacetylbaccatine III esterified at C-7 with a2,2,2-trichloroethoxycarbonyl group (TROC).

According to U.S. Pat. Nos. 5,917,062 and 6,020,507, the C-7 hydroxyl isprotected with carbobenzoxy (CBZ) or with carbo-t-butoxy (Boc), followedby selective acetylation of C-10 hydroxyl,

It is apparent from literature that a crucial aspect of paclitaxelsemi-synthesis is to selectively protect the hydroxyls on the diterpenmoiety (10-deacetylbaccatine III skeleton). The C-7 position is the mostreactive and is therefore functionalized with groups which are easy toremove subsequently. The most commonly used group is triethylsilyl(TES), which is stable under the conditions used for the esterificationof the other hydroxyls involved in the synthesis, and provides about 85%conversion yield. Approximately 85% yields are obtained when an acetylgroup is subsequently introduced at the C-10 position.

A novel process for the synthesis of paclitaxel has now been found,which provides higher final yields as well as other advantages comparedwith the known processes.

The process according to the invention comprises the following steps:

a) protection of the hydroxyls at the 7- and 10-positions of10-deacetylbaccatine III (10-DAB III),

 wherein R═R′=trichloroacetyl, or R′=acetyl and R is selected fromt-butoxycarbonyl and trichloroacetyl,

b) esterification of the hydroxyl at 13 with 3-phenyl-2-epoxypropionicacid

c) removal of the protective groups at the 7- and 10-positions (1) ifthey are both tichloroacetyl groups, followed by selective acetylationat the 10-position (2) and opening of the epoxide with sodium azide (3);

 or, alternatively,

c′) if R′=acetyl and R=trichloroacetyl, opening of the epoxide withsodium azide and simultaneous deprotection at the 7-position

d) reduction of the azido group to amino group

e) benzoylation to give the final product

The starting product is 10-deacetyl baccatine III (10-DAB III), which isextracted from the leaves of Taxus baccata. In the first step, 10-DABIII is quantitatively esterified at the C-7 and C-10 hydroxyls. WhenR═R′=trichloroacetyl, 10-DAB III is reacted with trichloroacetylchloride in methylene chloride in the presence of triethylamine and ofcatalytic amounts of 4-dimethylaminopyridine (DMAP). When R≠R′, first10-DAB III is selectively acetylated with acetic anhydride in thepresence of cerium, scandium, ytterbium salts, preferably CeCl3.7H20.The resulting baccatine III is subsequently protected at C-7 with at-butoxycarbonyl or trichloroacetyl group. The first can be introducedby reacting baccatine III with t-butoxy-pyrocarbonate in the presence ofDMAP and ethyldiisopropylamine or, alternatively, following theprocedure described in U.S. Pat. No. 5,917,062. The trichloracetyl groupcan be introduced at position 7 by reaction with trichloroacetylchloride in pyridine.

In the subsequent step (b), the hydroxyl at position 13 is esterifiedwith 3-phenyl-2-epoxypropionic acid, preferably with its ammonium saltin toluene in the presence of dicyclohexylcarbodiimide, DMAP andp-toluenesulfonic acid, thereby obtaining(2R,3R)-3-phenyl-2,3-epoxy-propionic acid baccatine III ester.

When both protective groups R and R′ are trichloroacetyl, they can beremoved using the conditions and reagents described by Zheng et al.,Tetrahedron Lett., 1995, 36, 2001, and by Datta et al., J. Org. Chem.,1995, 60, 761. Preferably, the two trichloroacetyl groups are removedwith two equivalents of ammonium hydroxide. The deprotected compound isselectively acetylated at position 10 with acetic anhydride in thepresence of cerium, scandium or ytterbium salts, preferably CeCl₃.7H2O.

The resulting compound is reacted with NaN₃ in aqueous methanol in thepresence of methyl formate, in the conditions reported in literature(Yamaguchi T., Tetrahedron Letters 39, 5575-78, 1998), to provide thecorresponding azide.

Alternatively, when R=trichloroacetyl and R′=acetyl (d), the oxiranereacts with NaN₃ to give the corresponding azide with deprotection atthe 7-position, corresponding to the compound obtained at step (c′).

The azide is reduced to amine in the subsequent step (d). The reductioncan be carried out with hydrogen on catalyst or with PPh₃. The productobtained at the last step (e) is benzoylated at the amino group to givepaclitaxel. Benzoylation can be carried out with benzoic anhydrideeither simultaneously to reduction or subsequently on the isolatedreduced product, using stoichiometric amounts of benzoyl chloride in thepresence of potassium carbonate.

The following examples illustrate the invention in greater detail.

EXAMPLE I Synthesis of 7-Trichloroacetyl-baccatine III

In a 25 ml round-bottom flask, 0.603 g (1.03 mmol, 1.0 eq) of baccatineIII were dissolved under magnetic stirring in 9.7 ml of dry pyridine at25° C. under nitrogen atmosphere. 138 μl (1.23 mmol, 1.23 eq) oftrichloroacetyl chloride were dropped into the clear pale yellowsolution. 30 min after completion of the addition, a white precipitateformed. Further 120 μl (1.07 mmol; 1 eq) of trichloroacetyl chloridewere dropped into the reaction suspension, under the same conditions asabove. After 20 min the solution had yellow-brown color. The almostcomplete conversion of the starting baccatine III was observed by TLC(SiO₂, n-hexane/EtOAc, 2:3). The reaction mixture was diluted withCH₂Cl₂. The resulting solution was repeatedly washed with a CuSO₄saturated solution, until pyridine had been completely removed (thesolution had no longer blue color). The organic phase was concentratedunder vacuum, dried over MgSO₄, filtered, and the solvent wasevaporated, to obtain 0.612 g of a white-yellowish powder correspondingto 7-trichloroacetyl-baccatine III, having the following spectroscopiccharacteristics.

¹H NMR (400 MHz, CDCl₃): δ_(ppm)=1.08 (s, 3H, Me), 1.13 (s, 3H, Me),1.86 (s, 3H, Me), 1.97 (ddd, 1H, J₁=14.4 Hz, J₂=10.3 Hz, J₃32 1.9 Hz,C6-H), 2.13 (d, 3H, J=1,2 Hz, Me), 2.15 (s, 3H, Me), 2.30 (s, 3H, Me),2.32-2.28 (m, 2H, C14-H₂), 2.68 (ddd, 1H, J₁=14.4 Hz, J₂=9.3 Hz, J₃=7.3Hz, C6-H), 4.04 (d, 1H, J=7.0 Hz, C3-H), 4.17-(dd, 1H, J₁=8.4 Hz, J₂=1.0Hz, C20-H), 4.34 (d, 1H, J=8.4 Hz, C20-H), 4.86 (t, 1H, J=7.5 Hz,C13-H), 4.98 (dd, 1H, J₁=9.5 Hz, J₂=1.7 Hz, C5-H), 5.65 (d, 1H, J=7.0Hz, C2-H), 5.70 (dd, 1H, J₁=10.4 Hz, J₂=7.4 Hz, C7-H), 6.42 (s, 1H,C10-H), 7.52-7.46 (m, 2H, arom), 7.62 (m, 1H, arom), 8.10 (m, 2H, arom);¹³C NMR (100 MHz, CDCl₃): δ_(ppm)=10.8, 15.5, 20.4, 20.9, 22.8, 26.9,32.5, 38.6, 43.0, 47.2, 56.2, 68.1, 74.5, 75.5, 76.5, 77.0, 79.0, 80.5,83.7, 89.9, 129.0, 129.4, 130.3, 132.0, 134.0, 145.4, 160.8, 167.2,169.2, 171.0, 201.9.

EXAMPLE II Synthesis of (2′R,3′R)-7-Trichloroacetyl-baccatineIII-13-(3′-phenyl-2′,3′-epoxypropionate)

0.164 g (1.00 mmol, 1 eq) of freshly prepared 3-phenyl-2-epoxypropionicacid were dissolved at 0° C. in 30 ml of anhydrous toluene. Subsequently0.5 g (1 mmol, 0.68 eq) of 7-(trichloroacetyl)-baccatine III[7-(TCA)-baccatine III] were added under nitrogen atmosphere at 0° C.Finally, dicyclohexylcarbodiimide (DCC, 0.21 g, 1.00 mmol, 1.0 eq),4-dimethylamino pyridine (DMAP, 0.084 g, 0.68 mmol, 0.66 eq) andp-toluenesulfonic acid (p-TSA, 0.17 g, 0.10 mmol, 0.1 eq) were added, insuccession. The solution was then heated at 70° C. under magneticstirring and nitrogen flow. The progress of the reaction was controlledby TLC (SiO₂, n-hexane/EtOAc, 3:2). The first spot, having R_(f)=0.28,corresponds to 7-(TCA)-baccatine III epoxy ester. The second spot,having R_(f)=0.11, corresponds to 7-(TCA)-baccatine III. After 3 hours,the mixture was cooled and the suspended solid was filtered. Theprecipitated dicylohexylurea (DCU) was washed with CH₂Cl₂. The combinedorganic fractions were concentrated to dryness. The resulting crude(0.919 g) was chromatographed by flash chromatography (SiO₂,n-hexane/EtOAc, 3:2). 0.100 g (0.14 mmol, 20%) of unreacted7-TCA-baccatine III and 0.435 g (0.49 mmol, 73%) of(2′R,3′R)-7-Trichloroacetyl-baccatineIII-13-(3′-phenyl-2′,3′-epoxypropionate) having the followingspectroscopic characteristics were obtained:

¹H NMR (400 MHz, CDCl₃): δ_(ppm)=1.11 (bs, 6H, 2Me), 1.25 (bs, 1H, OH),1.76 (d, 3H, J=1.2 Hz, Me), 1.84 (s, 3H, Me), 2.02-1.92 (m, 3H,C14-H₂+C6-H), 2.13 (s, 3H, Me), 2.39 (s, 3H, Me), 2.69 (ddd, 1H, J₁=14.6Hz, J₂=9.3 Hz, J₃=7.3 Hz, C6-H), 3.92 (d, 1H, J=6.9 Hz, C3-H)+, 3.97 (d,1H, J=4.7 Hz, C2′-H), 4.15 (dd, 1H, J₁=8.4 Hz, J₂=1.0 Hz, C20-H), 4.31(d, 1H, J=8.3 Hz, C20-H), 4.33 (d, 1H, J=4.7 Hz, C3′-H), 4.97 (dd, 1H,J₁=9.5 Hz, J₂=1.8 Hz, C5-H), 5.63 (d, 1H, J=6.8 Hz, C2-H), 5.65 (dd, 1H,J₁=10.7 Hz, J₂=7.33 Hz, C7-H), 6.02 (dt, 1H, J₁32 8.8 Hz, J₂=1.8 Hz,C13-H), 7.45-7.30 (m, 5H, arom), 6.34 (s, 1H, C10-H), 7.49 (m, 2H,arom), 7.64 (m, 1H, arom), 8.00 (m, 2H, arom); ¹³C NMR (100 MHz, CDCl₃):δ_(ppm)=10.8, 14.9, 20.8, 21.0, 22.5, 26.5, 32.4, 35.7, 43.2, 46.7,56.0, 56.1, 57.9, 70.9, 74.5, 74.8, 76.4, 76.7, 79.0, 80.6, 83.6, 89.8,126.8, 128.7, 128.9, 129.2, 129.3, 130.2, 132.6, 133.0, 134.1, 141.3,160.7, 166.3, 167.1, 169.1, 170.1, 201.3.

EXAMPLE III Synthesis of (2′R,3′R)-baccatineIII-13-(3′-azido-2′-hydroxy-3′-phenyl-propionate.

In a 25 ml one-necked round-bottom flask equipped with magnetic stirrer,0.397 g (0.45 mmol, 1 eq) of (2′R,3′R)-7-trichloroacetyl-baccatine III13-(3′-phenyl-2′,3′-epoxypropionate) were suspended at 25° C. in 10.0 mlof CH₃OH. 1.26 ml of H₂O, 1.26 ml of HCOOCH₃ and 0.735 g (11.3 mmol,25.0 eq) of sodium azide were added in succession. Temperature wasraised to 50° C. and the progress of the reaction was checked by TLC(SiO₂, CHCl₃/EtOAc/MeOH, 12.0:2.0:0.3). Disappearance of the startingproduct and simultaneous formation of two products having R_(f)=0.22 and0.29, respectively, were observed. The product having R_(f)=0.29 wassubsequently identified as the final product, whereas the product withR_(f)=0.22 was (2′R,3′R)-baccatineIII-13-(3′-phenyl-2′,3′-epoxypropionate) formed as a reactionintermediate. The product having R_(f)=0.29 growths in time to thedetriment of the product having R_(f)=0.22. The reaction solution after46 h had yellow brown color with a white precipitate (unreacted NaN₃).The reaction was quenched after 46 h by addition of water, two furtherspots were observed, with R_(f)0.38 and 0.13 (unrecovered decompositionproducts). The precipitated milky white solid was filtered, washed withwater and then with AcOEt. A diphasic mixture was obtained, both phasesbeing clear. The two phases were separated. The aqueous phase wasextracted three times with AcOEt and the combined organic phases wereconcentrated and dried over MgSO₄. The mixture was filtered and thesolvent was evaporated off, to obtain 0.335 g of a white-yellowishpowder. The resulting crude was purified by flash chromatography (SiO₂,CHCl₃/EtOAc/MeOH 12:2:0.3), to obtain 0.279 g (0.36 mmol; 80%;R_(f)0.22) of (2′R,3′R)-baccatine III-13-(3′-azido-2′-hydroxy-3′-phenyl-propionate).

The compound has the following spectroscopic characteristics:

¹H NMR (400 MHz, CDCl₃): δ_(ppm)=1.14 (s, 3H, Me), 1.25 (bs, 4H, Me+OH),1.67 (s, 3H, Me), 1.87 (ddd, 1H, J₁=13.9 Hz, J₂=11.1 Hz, J₃=2.5 Hz,C6-H), 1.93 (d, 3H, J=0.8 Hz, Me), 2.08 (d, 2H, J=8.8 Hz, C14-H₂), 2.24(s, 3H, Me), 2.26 (s, 3H, Me), 2.55 (m, 2H, C6-H+C7-OH), 3.28 (d, 1H,J=8.4 Hz, C2′-OH), 3.77 (d, 1H, J=7.2 Hz, C3-H), 4.15 (dd, 1H, J₁=8.2Hz, J₂=0.8 Hz, C20-H), 4.28 (d, 1H, J=8.2 Hz, C20-H), 4.41 (m, 2H,C7-H+C2′-H) 4.93 (dd, 1H, J₁=9.6 Hz, J₂=2.0 Hz, C5-H), 4.96 (d, 1H,J=4.4 Hz, C3′-H), 5.64 (d, 1H, J=7.2 Hz, C2-H), 6.17 (dt, 1H, J₁=7.9 Hz,J₂=1.2 Hz, C13-H), 6.30 (s, 1H, C10-H), 7.46-7.32 (m, 5H, arom),7.46-7.32 (m, 5H, arom), 7.50 (m, 2H, arom), 7.63 (m, 1H, arom), 8.06(m, 2H, arom); ¹³C NMR (100 MHz, CDCl₃): δ_(ppm)=9.8, 15.3, 21.1, 21.9,22.6, 27.0, 35.6, 35.8, 43.3, 45.9, 58.8, 68.1, 72.0, 72.4, 75.1, 75.3,75.8, 76.7, 79.4, 81.3, 84.6, 127.9, 128.9, 129.2, 129.5, 130.3, 133.4,134.1, 135.3, 142.2, 167.2, 170.5, 171.5, 203.8.

EXAMPLE IV Synthesis of N-debenzoyl-paclitaxel

In a 25 ml two-necked round-bottom flask, 0.102 g (0.13 mmol, 1.0 eq) of(2′R,3′R)-7-hydroxy-baccatineIII-13(3′-azido-2′-hydroxy-3′-phenyl-propionate) were dissolved in 5.2ml of freshly distilled CH₂Cl₂ and the resulting pale yellow solutionwas added with H₂O (0.05 ml), then with 0.071 g (0.26 mmol, 2.0 eq) ofPPh₃. The mixture was reacted at room temperature under magneticstirring. After 16 h the reaction was checked by TLC (SiO₂, CHCl₃/CH₃OH9:1). The starting product (R_(f)=0.61) had disappeared and a spot withR_(f)=0.19 was observed. The reaction was quenched by diluting themixture (of pale yellow color with white precipitate) with CHCl₃.Afterwards, the mixture was washed with distilled H₂O and then with asodium chloride saturated solution (brine). The bright yellow organicphase was dried over MgSO₄, then filtered and the solvent was evaporatedoff. 0.177 g of an ochre yellow oil were obtained. The crude wassubjected to flash chromatography (SiO₂, CHCl3/CH₃OH 9:1), to obtain0.074 mg (0.10 mmol; 76%) of N-debenzoyl-paclitaxel (pale yellowpowder).

¹H NMR (400 MHz, CDCl₃): δ_(ppm)=1.07 (s, 3H, Me), 1.09 (s, 3H, Me),1.38-1.22 (bs, 2H, 2OH), 1.75 (s, 3H, Me), 1.88 (s, 3H, Me), 1.90 (s,3H, Me), 1.93 (s, 3H, Me), 2.20-1.96 (m, 6H, C14-H₂+C6-H, NH2+OH), 2.52(ddd, 1H, J₁=15.7 Hz, J₂=9.5 Hz, J₃=5.9 Hz, C6-H), 3.88 (d, 1H, J=7.2Hz, C3-H), 4.10 (d, 1H, J=4.0 Hz, C20-H), 4.17 (d, 1H, J=4.0 Hz, C20-H),4.22 (d, 1H, J=8.0 Hz, C2′-H), 4.26 (d, 1H, J=8.0 Hz, C3′-H), 4.56 (dd,1H, J₁=11.6 Hz, J₂=6.9 Hz, C7-H), 4.84 (d, 1H, J=8.8 Hz, C5-H), 5.83 (d,1H, J=7.2 Hz, C2-H), 6.25 (t, 1H, J=8.0 Hz, C13-H), 6.51 (s, 1H, C10-H),7.20-7.00 (m, 8H, arom), 8.13 (m, 2H, arom); ¹³C NMR (100 MHz, CDCl₃):δ_(ppm)=9.8, 15.2, 21.1, 22.0, 22.7, 27.0, 30.0, 35.4, 35.8, 43.3, 45.9,58.7, 71.3, 72.3, 75.2, 75.8, 76.6, 79.3, 81.2, 84.6, 127.2, 128.5,128.9, 129.0, 129.4, 130.3, 133.1, 134.1, 142.6, 167.1, 170.4, 171.5,173.2, 203.9.

EXAMPLE V Synthesis of Paclitaxel

In a 10 ml round-bottom flask, 0.031 g (0.041 mmol, 1.0 eq) ofN-debenzoyl-paclitaxel were dissolved in 1.25 ml of AcOEt. The clearyellow solution was added with 1.25 ml of a NaHCO₃ aqueous saturatedsolution. 7.1 ml (0.064 mmol, 1.5 eq) of benzoyl chloride were droppedinto the resulting diphasic mixture, under strong magnetic stirring. Theprogress of the reaction was checked by TLC (SiO₂, CHCl₃/CH₃OH 9:1).After disappearance of the starting product, a single spot havingR_(f)=0.50 was observed. The reaction mixture was diluted with AcOEt.The organic phase was separated from the aqueous one, which wasextracted with AcOEt (three extractions). The combined organic phaseswere dried over MgSO₄, filtered and concentrated. The crude (0.037 g)was dissolved in a 1:1 mixture of CH₂Cl₂/ethyl ether, then n-pentane(0.030 g, 0.035 mmol, 86%) was added to precipitate paclitaxel, havingthe spectroscopic characteristics reported in literature.

EXAMPLE VI Synthesis of the(2′R,3′R)-7,10-bis-trichloroacetyl-10deacetyl-baccatineIII-13-(3′-phenyl-1′, 3′-epoxypropionate)

In a 100 ml round-bottom flask 0.178 g (1.09 mmol, 1.0 eq) of freshlyprepared 3-phenyl-2-epoxypropionic acid at 0° C. were dissolved in 30 mlof anhydrous toluene. In the resulting solution, under nitrogenatmosphere and at 0° C., 0.663 g (0.79 mmol, 0.73 eq) of7,10-bis-(trichloroacetyl)-10-deacetyl baccatine III[7,10-bis-(TCA)-10-DAB III] were suspended. Finallydicyclohexylcarbodiimide (DCC, 0.224 g, 1.09 mmol, 1.0 eq),4-dimethylaminopyridine (DMAP, 0.088 g, 0.72 mmol, 0.66 eq) andp-toluenesulfonic acid (p-TSA, 0.19 g, 0.11 mmol, 0.1 eq) were added, insuccession. The reaction was carried out in heterogeneous phase at 70°C. under magnetic stirring and nitrogen flow. The progress of thereaction was checked by TLC (SiO₂, n-hexane/EtOAc, 3:2). The first spothaving R_(f)=0.28 corresponds to 7,10-bis-(TCA)-10-DAB III epoxy ester.The second spot having R_(f)=0.15 corresponds to 7,10-bis-(TCA)-10-DABIII. After 3 hours the mixture was cooled and the suspended solid wasfiltered. The dark yellow precipitate was washed with CH₂Cl₂: theresidual white solid was DCU. The combined organic fractions wereconcentrated and the resulting solid was subjected to flashchromatography (SiO₂, n-hexane/EtOAc, 3:2). 0.63 g of(2′R,3′R)-7,10-bis-trichloroacetyl-10-deacetyl-baccatineIII-13-3′-phenyl-2′,3′-epoxypropionate were obtained.

¹H NMR (400 MHz, CDCl₃): δ_(ppm)=1.12 (s, 3H, Me), 1.14 (s, 3H, Me),1.76-1.60 (m, 2H, C6-H+OH), 1.81 (s, 3H, Me), 1.88 (s, 3H, Me),2.04-1.98 (m, 2H, C14-H), 2.41 (s, 3H, Me), 2.69 (ddd, 1H, J₁=14.5 Hz,J₂=9.3 Hz, J₃=7.3 Hz, C6-H), 3.89 (d, 1H, J=7.2 Hz, C3-H), 3.98 (d, 1H,J=4.0 Hz, C2′-H), 4.14 (d, 1H, J=8.0 Hz, C20-H), 4.32 (d, 1H, J=8.0 Hz,C20-H), 4.34 (d, 1H, J=4.0 Hz, C3′-H), 4.97 (d, 1H, J=7.6 Hz, C5-H),5.70-5.62 (m, 2H, C7-H+C2-H), 6.05 (dt, 1H, J₁=8.4 Hz, J₂=1.0Hz, C13-H),7.52-7.30 (m, 7H, ArH), 6.39 (s, 1H, C10-H), 7.45 (m, 1H, ArH) 7.99 (m,2H, ArH); ¹³C NMR (100 MHz, CDCl₃): δ_(ppm)=10.9, 15.1, 20.8, 22.6,26.3, 32.5, 35.6, 43.1, 46.7, 55.9, 56.5, 58.0, 70.8, 74.2, 76.4, 78.6,78.9, 80.5, 83.5, 89.5, 89.6, 126.8, 128.8, 129.0, 129.1, 129.4, 130.2,131.5, 132.5, 134.2, 143.3, 160.6, 161.1, 166.3, 167.0, 170.3, 199.5.

EXAMPLE VII Synthesis of (2′R,3′R)-10-deacetyl-baccatineIII-13(3′-phenyl-2′,3′-epoxypropionate)

In a 25 ml round-bottom flask, 0.174 g (0.18 mmol, 1.0 eq) of(2′R,3′R)-7,10-bis(TCA)-10-DAB III-13-(3′-phenyl-2′,3′-epoxypropionate)were suspended in 3 ml of CH₃OH. The resulting suspension was cooled to0° C. and 0.24 ml (0.36 mmol, 2.0 eq) of a 1.57 M NH₃ aqueous solutionwere dropped therein, under strong magnetic stirring. The reaction wascarried out for 15 min at 0° C., during which the suspension becameyellow-greenish. After that, the mixture was warmed to room temperatureand reacted for a further 5 min, to completely dissolve the precipitate,obtaining a clear yellow-greenish solution. The complete disappearanceof the starting compounds was checked by TLC (SiO₂, n-hexane/EtOAc,3:2), which gave a single spot on the baseline. The reaction mixture wasdiluted with H₂O to obtain a milky white solution, the organic phase wasextracted therefrom (3 extractions) with AcOEt (upon addition of theorganic solvent, an emulsion formed which was broken by dissolving NaCltherein). The combined organic phases were dried over MgSO₄, filtered,and the solvent was evaporated. 0.194 g of white powder of(2′R,3′R)-10-deacetyl-baccatine III-13-(3′-phenyl-2′,3′-epoxypropionate)were obtained.

¹H NMR (400 MHz, CDCl₃): δ_(ppm)=1.05 (s, 3H, Me), 1.09 (s, 3H, Me),1.71 (s, 3H, Me), 1.72 (d, 3H, J=1.2 Hz, Me), 1.83 (m, 1H, C6-H), 1.95(2H, d, J=8.8 Hz, C14-H₂), 2.34 (s, 3H, Me), 2.58 (ddd, 1H, J₁=14.6 Hz,J₂=9.9 Hz, J₃=6.9 Hz, C6-H), 3.85 (d, 1H, J=7.3 Hz, C3-H), 3.95 (d, 1H,J=4.4 Hz, C2′-H), 4.14 (d, 1H, J=8.4 Hz, C20-H), 4.22 (dd, 1H, J₁=11.3Hz, J₂=6.6 Hz, C7-H), 4.27 (d, 1H, J=8.4 Hz, C20-H), 4.31 (d, 1H, J=4.4Hz, C3′-H), 4.95 (d, 1H, J=8.8 Hz, C5-H), 5.16 (s, 1H, C10-H), 5.59 (d,1H, J=7.3 Hz, C3-H), 5.99 (dt, 1H, (d, 1H, J₁=8.8 Hz, J₂=1.2 Hz, C7-H),7.30-7.50 (m, 7H, arom), 7.60-7.70 (m, 1H, arom), 7.90-8.00 (m, 2H,arom).

EXAMPLE VIII Synthesis of (2′R,3′R)-baccatineIII-13-(3′-phenyl2′,3′-epoxypropionate)

A solution of (2′R,3′R)-10-deacetyl-baccatineIII-13-(3′-phenyl-2′,3′-epoxypropionate) (138 mg) in 3 ml of drytetrahydrofuran was added with 7.3 mg of CeCl₃.7H₂O and 0.073 ml ofacetic anhydride. The reaction mixture was stirred at room temperaturefor 5 hours, during which time the reaction mixture became homogeneous.1 g of ice was added, keeping under stirring for 1 hour. The organicsolvent was evaporated off under vacuum and the residue was diluted with5 ml of H₂O. The formed precipitate was filtered and dried under vacuumpump for 18 h. The resulting product (white powder, 130 mg) has thefollowing characteristics:

¹H NMR (400 MHz, CDCl₃): δ_(ppm)=1.05 (s, 3H, Me), 1.09 (s, 3H, Me),1.71 (s, 3H, Me), 1.72 (d, 3H, J=1.2 Hz, Me), 1.83 (m, 1H, C6-H), 1.95(2H, d, J=8.8 Hz, C14-H₂), 2.34 (s, 3H, Me), 2.58 (ddd, 1H, J₁=14.6 Hz,J₂=9.9 Hz, J₃=6.9 Hz, C6-H), 3.85 (d, 1H, J=7.3 Hz, C3-H), 3.95 (d, 1H,J=4.4 Hz, C2′-H), 4.14 (d, 1H, J=8.4 Hz, C20-H), 4.22 (dd, 1H, J₁=11.3Hz, J₂=6.6 Hz, C7-H), 4.27 (d, 1H, J=8.4 Hz, C20-H), 4.31 (d, 1H, J=4.4Hz, C3′-H), 4.95 (d, 1H, J=8.8 Hz, C5-H), 5.59 (d, 1H, J=7.3 Hz, C3-H),5.65 (dd, 1H, J₁=10.7 Hz, J₂=7.33 Hz, C7-H), 6.34 (s, 1H, C10-H),7.30-7.50 (m, 7H, arom), 7.60-7.70 (m, 1H, arom), 7.90-8.00 (m, 2H,arom).

EXAMPLE IX Synthesis of (2′R,3′R)-baccatineIII-13-(3′-azido-2′-hydroxy-3′-phenyl-propionate.

In a 25 ml one-necked round-bottom flask equipped with magneticstirring, 0.17 g (0.45 mmol, 1 eq) of (2′R,3′R) baccatine III13-(3′-phenyl-2′,3′were suspended at 25° C. in 5 ml of CH₃OH. 0.63 ml ofH₂O, 0.23 ml of HCOOCH₃ and 0.36 g (5.5 mmol, 12.5 eq) of sodium azidewere added in succession. The mixture was heated to 50° C. and theprogress of the reaction was checked by TLC (SiO₂, CHCl₃/EtOAc/MeOH,12.0:2.0:0.3). The reaction mixture after 46 h had yellow brown colorwith a white precipitate (unreacted NaN₃). H₂O (10 ml) was added and theprecipitated milky white solid was filtered, washed with water and thenwith AcOEt. The two phases were separated, the aqueous phase wasextracted three times with AcOEt and the combined organic phases wereconcentrated and dried over MgSO₄, filtered and the solvent wasevaporated off, to obtain 0.20 g of a white-yellowish powder. Theresulting crude was purified by flash chromatography (SiO₂,CHCl₃/EtOAc/MeOH 12:2:0.3), to obtain 0.140 g of (2′R,3′R)-baccatineIII-13-(3′-azido-2′-hydroxy-3′-phenyl-propionate).

The compound has the same spectroscopic characteristics as the compoundobtained in Example III.

What is claimed is:
 1. A process for the preparation of paclitaxel,which comprises the following steps: a) protection of the hydroxyls atthe 7- and 10-positions of 10-deacetylbaccatine III (10-DAB III),

 wherein -R═R′-trichloracetyl, or R′=-acetyl and R is selected fromt-butoxycarbonyl and trichloracetyl, b) esterification of the hydroxylat the 13-position with 3-phenyl-2-epoxypropionic acid

c) removal of the protective groups at 7 and 10 (1) if they are bothtrichloroacetyl groups, followed by selective acetylation at the10-position (2) and opening of the epoxide with sodium azide (3);

 or, alternatively, c′) if R′=acetyl and R=trichloroacetyl, opening ofthe epoxide with sodium azide and simultaneous deprotection at the7-position

d) reduction of the azido group to amino group

e) benzoylation to give the final product


2. A process as claimed in claim 1, wherein 10-DAB III is protected atthe 7- and 10-positions with a trichloroacetyl group by reaction withtrichloroacetyl chloride in methylene chloride in the presence oftriethylamine and catalytic amounts of 4-dimethylaminopyridine (DMAP).3. A process as claimed in claim 1, wherein 10-DAB III is firstacetylated at the 10-position by reaction with acetic anhydride in thepresence of cerium, scandium or ytterbium salts, and is subsequentlyprotected at the hydroxyl at 7- with a t-butoxycarbonyl ortrichloroacetyl group.
 4. A process as claimed in claim 1, wherein thehydroxyl at 13- is esterified with phenyl-2-epoxypropionic acid ammoniumsalt in toluene in the presence of dicyclohexylcarbodiimide (DCC), DMAPand p-toluenesulfonic acid.
 5. A process as claimed in claim 1, whereinthe protective groups R=R′-trichloroacetyl are removed with ammoniumhydroxide.
 6. A process as claimed in claim 1, wherein the epoxide isopened with NaN₃ in aqueous methanol in the presence of methyl formate.7. A process as claimed in claim 1, wherein the azide is reduced toamine with hydrogen on catalyst or with PPh₃.
 8. A process as claimed inclaim 1, wherein benzoylation at the last step is carried out withbenzoic anhydride either simultaneously to reduction or subsequently onthe isolated reduced product with benzoyl chloride in the presence ofpotassium carbonate.
 9. As reaction intermediates, the followingcompounds:

wherein R and R′ are as defined in claim 1 or hydrogen.